鍺/碳化矽超晶格以及非線性極化旋轉誘發鎖模於摻鉺或摻銩光纖雷射系統之研究

Abstract

本研究中，首先我們使用純飽和吸收體機制啟動1.6微米被動鎖模摻鉺光纖雷射系統。我們使用低真空電漿輔助化學氣相沉積法成長鍺以及富碳碳化矽的堆疊多層結構飽和吸收體，並以化學方法剝離鍺以及富碳碳化矽的堆疊飽和吸收體使之夾於兩個單模光纖接頭間。經由X射線光電子能譜以及拉曼散射光譜，可以得知在鍺層中的氧化問題較低以及在富碳碳化矽層中擁有較多的碳-碳鍵結。另外也可以經由穿透式電子顯微鏡觀察到明顯的17層分層結構以及總厚度為201.486奈米。此外由選區域衍射以及高倍率的穿透式電子顯微鏡影像可以得知在鍺層中存在明顯的220晶向的奈米結晶結構以及在碳化矽層中有堆疊的石墨結構。將鍺以及富碳碳化矽的堆疊飽和吸收體加入雷射共振腔體後，可以產生被動鎖模光纖雷射，其雷射脈衝寬度為708飛秒以及對應的光譜線寬為3.95奈米。另外，當藉由提升樣品的成長溫度，可以使得樣品有較完整的結晶，同時可以增進摻鉺光纖雷射的輸出表現。接著，我們使用非線性極化旋轉機制啟動2微米之摻銩光纖雷射鎖模脈衝輸出，並嵌入不同長度之正色散光纖進行腔內色散管理。在嵌入正色散光纖前，即操作摻銩光纖雷射在負色散區時，光固子鎖模脈衝具有最窄927飛秒之脈衝寬，最寬4.38奈米之光譜線寬的雷射輸出在高泵浦電流的條件下。可啟動鎖模的半波片調整角度為13度。由非線性極化旋轉機制產生的最大自振幅調變係數為6.210-2。當雷射操作在最大泵浦功率500毫瓦時，會有最多13個光固子形成，並且束縛在一脈衝波包內並且在腔內循環。在進一步嵌入8公尺的正色散光纖於雷射共振腔體後，即調整此摻銩光纖雷射於為負色散條件下，可以獲得最寬的光譜線寬為6.3奈米，對應到最窄640飛秒的脈衝寬度。最後，於摻銩光纖雷射腔中嵌入10公尺的正色散光纖，藉由適當調整腔內偏振，可以得到另外兩種不同的鎖模脈衝包含耗散光固子以及似噪脈衝之鎖模操作。在似噪脈衝模式工作時，光譜線寬最寬5.62 奈米之鎖模雷射脈衝可以被獲得；而在耗散光固子模式工作時，最寬為2.44奈米的光譜線寬可以被觀察到。最後，我們研究由鍺以及富碳碳化矽的堆疊飽和吸收體與非線性極化旋轉雙重機制所產生的混成鎖模摻銩光纖雷射。在單純的非線性極化旋轉效應下，藉由極化控制器的調整可以獲得光譜線寬最寬3.55奈米脈衝寬1.13皮秒的脈衝，並且擁有訊噪比為60 dB以及較高的載波振幅抖動(CAJ)為3.34%，藉由調整極化控制器中的半波片角度以優化脈衝序列，可以發現此時的可啟動鎖模角度為14度。在加入鍺以及富碳碳化矽的堆疊飽和吸收體後，因為非線性極化旋轉機制以及飽和吸收體的混成效應可使自振幅調變參數獲得優化，因此此時極化控制器中半波片的可鎖模調控角度可以提升至30度。此外，因為飽和吸收體的優化，可以獲得光譜線寬最寬為4.61奈米，相應的最窄脈衝為875飛秒，同時訊噪比以及載波振幅抖動也被優化到62.33 dB及1.74%，擁有非常高的脈衝穩定性。

Firstly, the passively mode-locked Erbium-doped Fiber Laser (EDFL) with its central wavelength located at about 1600 nm initial mode-locked by only the saturable absorber (SA) is experiment demonstrated. The Ge/C-rich SiC stacked superlattice SA is deposited by the low-vacuum plasma enhanced chemical vapor deposition (PECVD) and lift off by the buffered oxide etching (BOE) solution later on sandwiched between two signal mode fiber patchcords with FC/APC connector to serve as the SA that can insert into cavity. The the bonding type and element composition are analysed by the Raman scattering spectroscopy and the X-ray photoelectron spectroscopy (XPS), which reveal that Ge layer is rarely oxidized and the more sp2-orbital and sp3-orbital C-C bonds exist in the C-rich SiC. In more detail, transmission electron microscopy (TEM) is used to analysis the hierarchical structure which reveals that the total thickness of the SA is estimated as 201.486 nm within 17 layers. From both high-resolution TEM image and the selected area diffraction (SAD) pattern, the d-spacing can be estimated as 0.21 nm and 0.34 nm that is corresponding to the (220)-oriented Ge and the (002)-oriented graphite interlayer, respectively. In additional, from the nonlinear transmittance measurement with incident different power, the self-amplitude modulation (SAM) coefficient of the SA is caculated as 0.001. However, such a relatively low SAM coefficient of the Ge/SiC stacked superlattice SA can provide enough mode-locked force for self-starting the passively mode-locked EDFL with purely SA induced SAM. The broadest optical spectra linewidth and the narrowest pulsewidth of the passively EDFL with the Ge/SiC stacked SA are 3.945 nm and 708 fs, respectively. At the last, the different growth temperature of the vapor deposition process of the superlattice SA is been compared and the higher growth temperature SA has the better performance of the EDFL due to the enhance crystallinity within the thin film deposited by PECVD. Secondly, the dispersion managed thulium-doped fiber laser (TDFL) passively mode-locked with NPR effect is demonstrated for sub-picosecond soliton pulsation at 2 m. The NPR induced polarization dependent loss (PDL) in polarized isolator provides the conventional soliton with linewidth, pulsewidth, and repetition frequency of 4.38 nm, 927 fs, and 18.9 MHz, respectively. The polarization angle tolerance for the NPR mode-locking of the TDFL is 13, indicating that NPR dependent maximal SAM is only 6.210-2. The carrier amplitude jitter of the TDFL soliton is reduced to 1.8% with corresponding signal-to-noise ratio increasing to 66.7 dB. When enlarging the pumping power to 500 mW, the tightly bunched soliton pulse packet in the TDFL with negative GDD occurs with 13 solitons inside the bunched envelope. By inserting normal dispersion fibers (NDFs) with different lengths, the group-velocity dispersion in TDFL can be detuned from negative to positive value for conventional and disspative soliton generation. After inserting the NDF with length of 8 m, the TDFL is mode-locked with shortened pulsewidth of 640 fs and broadened linewidth of 6.3 nm under slightly negative GDD condition. In more detail, the compensation ratio for the TDFL pulse is decreased from 0.548 to 0.139 when detuning the GDD in the intracavity from -0.803 ps2 to -0.08ps2. At the last, two mode-locking pulses of the noise-like and dissipative soliton pulses for the passively mode-locked TDFL with the positive group delay dispersion are also observed by inserting the 10-m-length NDF and suitably adjusting the polarization controller. The maximum spectrum linewidth of the noise-like soliton and the dissipative soliton are 5.62 nm and 2.44 nm, respectively, with the corresponding CAJ of 3.044% and 3.326%. Finally, the passively TDFL with the central wavelength located at 1950 nm mode-locked by hybrid mechanism incorporate with the NPR effect and the Ge/SiC stacked SA is demonstrated. The broadest optical spectrum and the narrowest pulsewidth that can be obtain from the 5% output port are 3.55 nm and 1.13 ps, respectively, with the solely NPR mode-locked mechanism under the pumping power is set at 500 mW. Moreover, the spectrum analyser is employed to analysis the stability of the pulse train provide by TDFL. The SNR is measure as 60 dB and the CAJ of the passively mode-locked TDFL without inserting the Ge/SiC stacked SA is estimated as 3.34%. However, the performance can be improve by inserting the Ge/SiC stacked SA into the cavity. It reveal that the optical spectrum linewidth, the corresponding pulsewidth, SNR and CAJ can be enhance to 4.61 nm, 875 fs, 62.33 dB and 1.74%, respectively, with the hybrid mode-locked mechanism. In more detail, the allowable mode-locked angle also can be improved form 14-degree to 30-degree with incorporating the NPR and the Ge/SiC stacked SA.